1. Field of the Invention
The present invention relates to an optical recording medium and a method of manufacturing such optical recording medium. More particularly, the present invention is preferable for an application to an optical recording medium on and from which an information signal is recorded and/or reproduced, by radiating a laser beam from a side on which a light transmitting layer is formed, to an optical disc having a recording layer and a light transmitting layer formed on a substrate.
2. Description of the Related Art
In recent years, various researches and developments with regard to an optical information recording method have been promoted in a field of information recording. This optical information recording method has merits that a recording operation and/or a reproducing operation can be done without any direct contact to a medium, and that it is possible to attain a recording density higher by one digit or more than that of a magnetically recording method. Also, this optical information recording method has a further merit that it can be applied to respective memory forms such as a read only type, a write once type (recordable type), a rewritable type and the like. For this reason, the applications to wide services from industrial use to consumer use are considered as a method which can attain a file that is low in cost and large in capacity.
Among them, a digital audio disc (DAD), an optical video disc and the like, which are the optical discs corresponding to the memory form of the reproduction only type, are widely prevalent, in particular.
The optical disc such as the DAD or the like has the configuration that a reflection film constituted by a metallic thin film, such as an aluminum (Al) film and the like, is formed on a transparent disc substrate on which concave and convex patterns such as a pit and a groove indicating an information signal and the like are formed, and that a protective film for protecting the reflection film from water (H2O) and oxygen (O2) in the air is formed on the reflection film. And, when the information signal is reproduced from the optical disc, a reproducing light such as a laser beam or the like is radiated to the concave and convex patterns from the optical disc substrate side. Accordingly, the information signal is detected on the basis of a reflectance difference between an incident (incoming) light and a reflected light of the reproducing light.
When such an optical disc is manufactured, an optical disc substrate having concave and convex patterns is firstly fabricated by an injection molding method. Next, a reflection film constituted by a metallic thin film is formed on the optical disc substrate by a vacuum evaporation method. Subsequently, a film formed by coating of an ultraviolet curable resin on an upper layer of the reflection film is used as the protective film.
Here, in the above-mentioned method of recording the optical information, the higher recording density has been requested in recent years. So, in order to satisfy the request of the higher recording density, a technique is proposed for increasing a numerical aperture (NA) of an objective lens used at a time of a radiation of a reproducing light of an optical pickup and thereby reducing a spot diameter of the reproducing light.
That is, NA of an objective lens used at a time of a reproducing a conventional DAD, that is, reading information recorded in the conventional DAD, is 0.45. On the contrary, NA of an objective lens used at a time of reproducing an optical video disc such as a DVD (Digital Versatile Disc) having a recording capacity equal to 6 to 8 times that of the DAD is about 0.60. In this way, it is possible to reduce the spot diameter.
However, if a higher NA is advanced in such an objective lens, it is necessary to thin a disc substrate in an optical recording medium, in order to transmit the radiated reproducing light. This is because an allowable amount of an angle (a tilt angle) deviated from a vertical portion of a disc plane with respect to an optical axis of the optical pickup is dropped, and the tilt angle easily receives the influence of double refraction and aberration caused by a thickness of the disc substrate. Thus, trying to have a thinner disc substrate results in a tilt angle to be as small as possible. For example, in the above-mentioned DAD, the thickness of the disc substrate is set at about 1.2 mm. On the contrary, in the optical video disc having the recording capacity equal to 6 to 8 times that of the recording capacity of the DAD, such as the DVD or the like, the thickness of the disc substrate is set at about 0.6 mm.
However, in consideration of a request for the further higher recording density in future, it is necessary to further reduce the thickness of the disc substrate. So, there is proposed an optical recording medium in which concave and convex portions are formed on a major surface of a disc substrate, as an information signal area; a reflection film and a light transmitting layer of a thin film to transmit a light on the information signal area are sequentially laminated; and a reproducing light from the light transmitting layer side is radiated to the disc substrate so as to reproduce an information signal. Such an optical recording medium can cope with the higher NA of the objective lens by reducing the thickness of the light transmitting layer.
One example of the optical recording medium designed to reproduce the information signal by radiating the reproducing light from the light transmitting layer side is disclosed in Japanese Patent Application Laid-Open No. Hei 10-283683. In this application, when a light transmitting layer is formed, a method is employed for laminating light transmitting sheets on a disc substrate by using an ultraviolet curable resin.
According to the disclosure of the above-mentioned Japanese Patent Application Laid-Open No. Hei 10-283683, the ultraviolet curable resin is firstly fed onto a major surface of a disc substrate. The light transmitting sheet configured so that a laser beam can be transmitted therethrough is placed on the ultraviolet curable resin. Next, the ultraviolet curable resin is filled between the substrate and the light transmitting sheet by rotating in an in-plane direction the light transmitting sheet and the substrate laminated having the ultraviolet curable resin disposed therebetween. At a stage when the ultraviolet curable resin is sufficiently filled between the substrate and the light transmitting layer, the ultraviolet is radiated onto the resin, and the resin is cured. Accordingly, the substrate and the light transmitting sheet are adhered to each other. As mentioned above, the light transmitting layer which comprises the ultraviolet curable resin and the light transmitting sheet is formed.
In the above-mentioned optical disc, various experiments and considerations have been progressed for practical use. However, as a result of the various experiments by the inventor with regard to such an optical recording medium, it is confirmed, in the above-mentioned conventional optical disc, that when the optical disc is chucked and rotated at a predetermined rotation speed, a deflection (run out) of the disc becomes very large. In the present specification, the term “deflection” is used for express the phenomenon of vertical deviation, run-out or the like of the disc surface.
Such a deflection brings about a recording defect and a reproduction defect, in a case of an optical recording medium which has a higher NA of the objective lens of the optical system and a shorter wave length of the laser beam used in the recording and/or the reproduction. For this reason, it results in a severe problem in a practical application of the optical recording medium having the improved recording density.
There is a need for an optical recording medium having a light transmitting layer formed on a disc substrate thereof, which has less deflection and on which a recording operation and/or a reproducing operation can be excellently carried out, and a method of manufacturing the same.